The present invention relates to a brake control system of a vehicle, and more particularly, to a brake control system which employs a phase-locked loop (PLL) scheme and is primarily used in a fast moving vehicle to stop the vehicle in a short distance while prevent the vehicle from skidding.
As will be hereinafter illustrated, the present invention is specifically referred to as an "Anti-Skid Brake System (ABS)" and which is primarily used in an aircraft with landing gears including a set of nose wheels and a set of main wheels. The purpose of the present invention is to provide the aircraft with a brake control system which controls the brake force applied to the main wheels such that the aircraft, when landing, would come to a stop in a shortest distance while prevent the aircraft from skidding out of the runway.
In a landing aircraft, the brake force is generally only applied to the main wheels while the nose wheels undergo free rolling. If a brake force is overly applied to the main wheel, the main wheel may undergo a slip. It is therefore the primary object of the present invention and the aforementioned patents to apply an appropriate braking force to the main wheel. It is generally known that slip of the wheels will occur if the brake force is overly applied to the main wheels. Should this condition takes place, the effects of the braking would be deteriorated and the high-speed moving aircraft may go through a dangerous situation as to skid out of runway. Consequently, the braking mechanism has to be controlled to apply an appropriate braking force to the main wheels. The optimal braking force is such that it would stop the landing aircraft in a shortest distance while slip would not be caused to the wheels.
Various control systems have been devised for the anti-skid brake control system. In U.S. Pat. No. 2,914,359, Yarber disclosed an "Anti-Skid Brake Control System" which mainly used mechanical devices to prevent vehicle skidding. In U.S. Pat. No. 2,906,376, Zeigler disclosed a "Non-skid Braking System Using Pre-set Pulsing Action" which measured the angular deceleration of the aircraft wheels and accordingly control a solenoid valve to modulate the braking force. And in U.S. Pat. No. 3,245,727, Anderson et al disclosed an "Anti-Skid Brake Control System" which measured the wheel slip as a feedback signal to control a hydraulic servo-valve.
In addition to the aforementioned patents, numerous engineering papers have been published discussing other schemes for the anti-skid brake control system. In summary, these control schemes utilize feedback signals including the angular deceleration of the main wheel, the slip, and the slip-velocity for modulating the braking force on the main wheels of the aircraft.
The phase-locked loop concept, which was originated from the field of radio communications, is rapidly expanding into other areas of application such as speed control of DC motors. The main idea of PLL is to synchronize the output signal with a reference signal in frequency as well as in phase. Referring to FIG. 1, the output signal of a VCO 12 is fed back to a phase detector 10 which compares with the feedback signal with the input signal. The phase error generated thereby from the phase detector 10 is first filtered by a low-pass loop filter 11 to retain average voltage, and then the PLL circuit forces the output frequency of the VCO 12 to change in a direction that reduces the frequency difference between the feedback signal and the input signal. In the synchronized state, often called locked state, the phase error between the output signal of the VCO 12 and the reference signal is fixed. Therefore, the frequency of the reference signal is locked.